Starch transitions of different gluten free flour doughs determined by dynamic thermal mechanical analysis and differential scanning calorimetry

Carbohydrate Polymers 127 (2015) 160–167

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Starch transitions of different gluten free flour doughs determined by dynamic thermal mechanical analysis and differential scanning calorimetry R. Moreira ∗ , F. Chenlo, S. Arufe Departamento de Enxe˜ naría Química, Escola Técnica Superior de Enxe˜ naría, Universidade de Santiago de Compostela, Campus Vida, 15782 Santiago de Compostela, Spain

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 21 January 2015 Received in revised form 13 March 2015 Accepted 14 March 2015 Available online 30 March 2015 Keywords: DMTA DSC Gelatinization Amylose Amylopectin Melting

Gluten-free flour doughs (three from different maize varieties and one from chestnut fruit) processed at the same consistency level (1.10 ± 0.07 N m) with different water absorption were used to determine the starch transitions by means of two different experimental techniques, differential scanning calorimetry (DSC) and dynamic thermal mechanical analysis (DMTA). The ranges of temperatures of gelatinization (G), amylopectin melting (M1), amylose–lipid complexes melting (M2) and amylose melting (M3) for all tested flour doughs were determined by both experimental techniques with acceptable agreement between them. The starch transitions in DMTA were determined by means of the elastic modulus (G, M1 and M2) or damping factor (G, M3) evolution with temperature. The temperatures and enthalpies of the transitions depended on water content, the nature and characteristics (mainly damaged starch) of the starch and the presence of other compounds (mainly lipid and sugars) in the flour doughs. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Gluten-free products based on starchy foods are in increasing demand due to the growing number of people diagnosed with coeliac disease. The research on the transformations experienced by food materials during thermal processes is essential to estimate and control the final product properties. The use of traditional and global starch source like maize or alternative and local source like chestnut is necessary to increase the supply of products with high quality to a growing market. The main component of maize and chestnut is the starch. Starch is present in form of granules with crystalline and amorphous structures. Physicochemical properties and thermal behaviour of starch depend on the amorphous/crystalline ratio and the arrangement of the structure in the granule (Tahir, Ellis, & Butterworth, 2010). Chemically, starch is constituted by two carbohydrate polymers, amylose (linear) and amylopectin (branched) (French, 1984) with different behaviour during thermal processing and also depending on the water content. Differential scanning calorimetry (DSC) is the most common method to study the thermal behaviour of isolated starches for

∗ Corresponding author. Tel.: +34 851816759. E-mail address: [email protected] (R. Moreira). http://dx.doi.org/10.1016/j.carbpol.2015.03.062 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

determining temperature of transitions and the corresponding enthalpies (Eliasson, 1980). Particularly, gelatinization of different starch is well studied in the bibliography by its importance in starch processing for food and non-food purposes. At high water content one broad endothermic peak, G, by the swelling of the amorphous region and subsequent melting of crystallites is observed, but at intermediate water content, this transition is partially postponed to higher temperatures resulting M1 transition (Jang & Pyun, 1996). Other thermal transitions, due to biopolymer interactions, can be determined at higher temperatures as the reversible dissociation of lipid–amylose complexes in the range from 100 to 120 ◦ C (Liu, Yu, Xie, & Chen, 2006; Torres, Moreira, Chenlo, & Morel, 2013) and also melting amylose above 140 ◦ C (Jang & Pyun, 1996). These transitions also depend on water content of the sample. Nevertheless, some endothermic peaks associated to the thermal transitions are very weak and consequently their determination and evaluation is troublesome. This fact is pronounced in samples with high water content because the limited dry mass amount and DSC is not sensitive to changes affecting to mechanical properties of the material (Warren, Royall, Butterworth, & Ellis, 2012). The study of the starch transitions in cereal doughs like maize and chestnuts flour doughs is more complex than the study of isolated starch from different sources. The presence of other biopolymers in a relevant proportion like proteins and lipids

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together with the particle size of the flour affect significantly the water absorption of the samples to achieve a determined consistency. At these conditions, doughs can be submitted to different industrial thermal processes involving operations such as baking, extrusion of flour based products and the material properties depend on starch gelatinization and other transitions promoted by high temperatures. The chemical, physical and viscoelastic properties of the final starchy products depend mainly on the extension of water–starch and water–biopolymers interactions developed during processing. Dynamic thermo-mechanical analysis (DMTA) is an experimental method in which a sinusoidal force is applied to the sample at fixed angular frequency measuring the stress and strain inside the linear viscoelasticity region (LVR) at constant heating/cooling rate. This analysis was employed by other authors to evaluate the starch gelatinization due to strong structural changes takes place during the plasticizing process promoted by water (Bogracheva, Wang, Wang, & Hedley, 2002). In fact, gelatinization temperatures range can be clearly observed by peaks of the storage modulus, G , complex viscosity, G* or tan ı (Chanvrier, Appelqvist, Li, Morell, & Lillford, 2013; Moreira, Chenlo, & Torres, 2011). The experimental determinations of the phase transitions at high temperatures (above 100 ◦ C) of flour doughs by using DMTA or DSC methods may lead to different results, beyond the physical fundamentals of each technique, by the fact of the water evaporation. Water during DSC is evaporated generating an overpressure inside the sealed pan and equilibrium between sample and surrounding is achieved. In DMTA experiments, water is also removed by evaporation and sample dries. Both methods are interesting because DSC analysis can be useful to understand the crumb formation and DMTA tests can be related to the crust generation (Rouille, Chiron, Colonna, Della Valle, & Lourdin, 2010; Vanin, Michon, Trystram, & Lucas, 2010). Furthermore, the water content effect on the determination of the transitions by both methods can be also evaluated. The aim of this work is to determine the starch transitions of several gluten-free flour doughs (three from different maize varieties and one from chestnut fruit) processed at the same consistency level with different water absorption by two different experimental techniques (DSC and DMTA) in order to establish comparisons between the results obtained with both tested methods. Results on starch transitions are discussed in relation to chemical and physical properties of flours and doughs also experimentally determined. 2. Materials and methods 2.1. Materials Maize (0.40 ± 0.05 kg water/kg dry solid, dry basis (d.b.)) and chestnut (1.29 ± 0.13 d.b.) flours obtained from 3 different types of Spanish maize kernels, white (WM, Rebordanes variety), yellow (YM, Sarreaus variety) and purple (PM, Meiro variety) and chestnuts (CH, Castanea sativa Mill.), acquired in a local market were employed as raw material. 2.2. Methods 2.2.1. Flour processing Chestnut fruits, previously selected, dehulled, peeled and cut in cubes using a laboratory blender (Waring, Model HGBTWT, USA) and maize kernels were air-dried in a pilot-scale tray dryer (Angelantoni Challenge 250, Italy) at 45 ◦ C with air velocity of 2 m/s, relative humidity of 30% and 5–6 kg/m2 of load density. Drying was carried out until a moisture content of the sample of 11% d.b. was achieved. Final dry solid was determined gravimetrically after sample drying by using a vacuum oven (Heareus Vacutherm 5250

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VT6025) at 70 ◦ C and 0.15 atm of absolute pressure to dry up to constant weight (AOAC, 1995). Dried particles were milled using an ultra-centrifugal mill (ZM200 Retsch GmbH) with an internal sieve of 200 ␮m. The flours obtained were placed in a desiccator with a saturated solution of Mg(NO3 )2 , prepared according to recommendations (Greenspan, 1977) to obtain a constant relative humidity of surrounding air of 54% at 25 ◦ C, until equilibrium between samples and surrounding air was reached. Equilibrated flours achieved constant moisture content (8–10%, d.b.). Flours were then storage at 4 ◦ C in vacuum sealed bags until its utilization. 2.2.2. Physicochemical characterization The average particle size of the obtained flours was determined by sieving employing standard sieves of 40, 63, 80, 125, 200 and 250 ␮m (Standard ISO-3310.1, Cisa Cedacería Industrial, Spain). Average particle diameter by mass (Dw ) was calculated considering the average particle size, Dpi (␮m), of each mass fraction, wi (%) . Starch characterization was carried out by means of total starch (TS, % g starch/g dry flour) and damaged starch (DS, % g damaged starch/g dry flour). TS was measured as total starch in flour without previous gelatinization using a “Total Start Assay Kit” whose method was approved by the American Association of Cereal Chemists (AACC, 2000). DS was determined as the starch fraction that is thermal or mechanically damaged, using a “Starch Damage Kit” (ICC, 1996). The starch extraction from maize and chestnut flours was carried out according to the method of Singh and Singh (2001) with minor modifications. Maize and chestnut powder (10 g) was added into 100 ml of distilled water with 0.5% (w/w) of sodium sulphite. The slurry was filtered through a 63 ␮m sieve. The residue on the sieve was washed with distilled water for three times. The filtrate was precipitated over night at 4 ◦ C. Then, the supernatant was discarded and the starch was washed with distilled water and precipitated again for two times. The starch was collected and dried at 40 ◦ C up to constant weight. The amylose/amylopectin ratio was determined according to the procedure previously established in bibliography (McGrance, Cornell, & Rix, 1998). All these tests were made at least in duplicate. Lipid content of flours was determined following ISO standards (ISO, 1982). Total fibre content of flours was evaluated by means of a standard enzymatic-gravimetric method according to AOAC (AOAC, 1996). Flours protein amount was established by Kjeldahl method (AOAC, 1996). Sugar content was determined by HPLC according to the AACC standard method (AACC, 1994). The assays were performed at least in triplicate. 2.2.3. Dough processing Doughs were obtained using Mixolab® apparatus (Mixolab® Chopin Technologies, France). The protocol utilized (ICC, 2008) consisted of flour and water mixing at constant temperature (30 ◦ C) and mixing rate (80 rpm) until the torque produced by dough (consistency) of 1.10 ± 0.07 N m, the same consistency reached by wheat flours in industrial dough elaboration, was achieved. At this consistency, the dough mixing properties like water absorption (WA), development (DT) and stability time (ST) were determined. The WA (% d.b.) is defined as the amount of water needed to obtain a dough with the desired consistency. The DT and ST are defined as the time to reach the maximum torque and the time at which the torque produced by dough is kept at 1.10 ± 0.07 N m, respectively. More details about the Mixolab® protocol and the characteristic parameters were previously reported (Moreira, Chenlo, Torres, & Prieto, 2010). 2.2.4. Differential scanning calorimetry (DSC) DSC studies of flour dough samples at the same water content of doughs studied by DMTA were prepared using the same method

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described in bibliography (Liu et al., 2006). The solid (∼1 g), in a glass vial, was weighted in an analytical balance (Denver Instruments SI-234), the water needed to obtain the desired WA was added with a micropipette and mixed well with a small spatula. WA levels were calculated by weight. The glass vial was sealed and the mixture was equilibrated for 24 h at room temperature. Then, a small portion of the sample (12.6–16.2 mg) was introduced in a steel pan, sealed with a lid and equilibrated at room temperature for 1 h. Thermal properties of these samples were studied using a differential scanning calorimetry (TA Q200, TA Instruments, New Castle, USA). An empty steel pan was used as reference. The sample pan and the empty pan were placed in two identical compartments of the furnace unit. The assay realized consisted of equilibrating the sample during 5 min at 40 ◦ C and then heating to 180 ◦ C at a constant heating rate of 4 ◦ C/min. All temperatures and enthalpy changes associated with thermal transitions observed in each endothermic curve were determined using TA Instruments Universal Analysis 2000 Software (version 4.7A, TA Instruments Waters LLC, New Castle, USA). 2.2.5. Thermo-mechanical analysis (DMTA) Flour doughs at the target consistency were tested in a controlled stress rheometer (MCR 301, Anton Paar Physica, Austria) equipped with a chamber (CTD 450, Anton Paar Physica, Austria) using parallel plates (50 mm diameter, 2 mm gap). The rheological assays were performed in the linear viscoelastic region of the doughs (0.1% of strain, 1 Hz of frequency). Temperature was increased from 30 to 180 ◦ C with a constant heating rate of 4 ◦ C/min. Values of storage modulus (G ), loss modulus (G ) and damping factor (tan ı) were determined to analyse the temperatures associated with thermal transitions using Rheoplus/32 software (version 3.21, Anton Paar, Ostfildern, Germany). All rheological assays were performed at least in duplicate. 2.2.6. Statistical analysis Experimental data were statistically analysed. Differences among means were identified by one-factor analysis of variance (ANOVA), followed by the Scheffe test and considering significant P-values ≤ 0.05 (IBM SPSS Statistics 22). 3. Results and discussion 3.1. Physicochemical properties In maize flours, the maximum mass fraction (from 28.0% to 50.3%) of particles was from 80 to 125 ␮m of particle size, Table 1. WM flours also showed an important population (27.0%) in the range of 125 to 200 ␮m. Maize flour particles with size between 63 and 80 ␮m were the second more important (29.8%) in YM flour while PM flour showed two fractions with high weight percent; 40–63 ␮m and 125–200 ␮m, with 27.7% and 23.4%, respectively. In spite of the different distributions of each tested flour, the average particle diameter by mass varied in a restricted interval

Table 1 Particle size distribution and average particle size in mass of tested flours.a Fraction (␮m)

Dpi (␮m)

200 < x < 250 125 < x < 200 80 < x < 125 63 < x < 80 40 < x < 63 x < 40 Average particle size, Dw (␮m)

225.0 162.5 102.5 71.5 51.5 20.0

wi , mass fraction (%) YM

WM

PM

CH

2.2 12.3 34.9 29.8 13.5 7.4 90.3

2.1 27.0 50.3 11.7 8.0 0.9 112.8

2.3 23.4 28.0 6.5 27.7 12.0 93.3

3.0 8.2 4.9 10.0 73.9 37.8

a Standard deviations of mass fraction were ±0.10. Yellow (YM), white (WM) and purple (PM) maize flour doughs and chestnut (CH) flour doughs.

(90.3–112.8 ␮m) with higher values for WM flours due to its higher hardness. CH flour showed an average particle size (37.8 ␮m) smaller than maize flours, indicating that chestnut is a softer material than maize. The most important mass fraction (73.9%) of CH flour was smaller than 40 ␮m. Chemical composition (% w/w, d.b.) of tested maize flours, YM, WM and PM, do not show great differences between them. Lipid content varied from 3.3 ± 0.2 up to 4.8 ± 0.7, total fibre content between 1.9 ± 0.4 and 3.7 ± 0.5, protein content between 6.0 ± 0.4 and 7.6 ± 0.2 and sugars content was practically invariant, 1.8 ± 0.6. CH flour show lower lipid (1.4 ± 0.2) content, intermediate protein (6.7 ± 0.1) content and higher total fibre (4.5 ± 0.1) and specially sugar (16.4 ± 0.5) content than maize flours. Total starch corresponds to the starch amount without gelatinize before the analysis. Total starch of the maize flours varied from 68.1 to 75.2 (% w/w, d.b.), Table 2. These values are in concordance with those reported by other authors for different maize flours: 66.9–74.1% (Hasjim, Srichuwong, Scott, & Jane, 2009) and 77.5% (Malumba et al., 2015). CH flour showed lower total starch content (50.9%) than maize flours due to mainly its high sugar content. In both cases, high total starch content were found and it can be related to the mild conditions during processing, particularly, the use of low drying temperatures (45 ◦ C) that avoided the occurrence of starch gelatinization. Damaged starch measures the starch fraction that is thermal or mechanically modified during processing (mainly drying and milling operations). Damaged starch varied from 7.3% to 25.0% (% w/w, d.b.) for all tested flours, Table 2. Same values were previously obtained for maize, 10.1–17.4% (Bolade, Adeyemi, & Ogunsua, 2009), and chestnut flour (Torres, Moreira, Chenlo, Morel, & Barron, 2014) with similar particle size. CH and YM flours showed the lowest values of damaged starch and WM and PM flours the highest. This last result can be explained by the highest resistance to the milling by WM than the other flours and starch granules are mechanically disturbed during milling. The opposite behaviour was obtained with CH flour. Amylose/amylopectin ratio of starch from tested sources varied in a narrow interval among (17.8 ± 0.7 for PM and 19.5 ± 0.3 for CH). Consequently, this ratio value cannot be considered in order to

Table 2 Total and damaged starch of tested flours and mixing curves parameters obtained in Mixolab® apparatus (target torque 1.10 ± 0.07 N m) of flour doughs.a

Total starch (% w/w, d.b.) Damaged starch (% w/w, d.b.) Mixing curves parameters Water absorption, WA (%, d.b.) Development time, DT (min) Stability time, ST (min)

YM

WM

PM

CH

68.1 ± 2.2a,b 8.6 ± 0.2a

73.8 ± 2.2a,b 25.0 ± 2.1b

75.2 ± 6.3b 18.2 ± 0.5b

50.9 ± 3.1a 7.3 ± 0.8a

63.0 ± 1.0b 0.88 ± 0.06b 1.97 ± 0.08d

90.0 ± 2.0d 0.75 ± 0.03a,b 1.50 ± 0.03c

81.1 ± 1.4c 0.73 ± 0.07a,b 1.05 ± 0.04b

52.9 ± 0.5a 0.50 ± 0.01a 0.43 ± 0.01a

a Data are the mean value and ±standard deviation. Yellow (YM), white (WM) and purple (PM) maize flour doughs and chestnut (CH) flour doughs. Data value with different letters in rows are significantly different, P ≤ 0.05.

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discuss differences related to starch transitions between different samples. 3.2. Pasting characteristics Flour doughs were made in Mixolab apparatus with target torque of 1.10 ± 0.07 N m. From the mixing curves some valuable parameters such as WA, DT and ST were obtained, Table 2. The water amount necessary to achieve the target torque depended on damaged starch in the flour. In fact, an acceptable linear relationship (R2 > 0.95) between both variables was found for all tested flour doughs, Fig. 1. CH doughs showed slightly less water absorption than expected due to probably the sucrose inhibition of granular starch hydration (Lund, 1984). The presence of damaged starch gives as result thicker doughs at the same water absorption. All samples showed low development times (<0.90 min) up to reach the target consistency and a broad stability range from 0.43 min of CH flour dough up to 1.50–1.97 min of PM and WM flour doughs. Stability depended greatly on average particle size of flours and consequently the very short stability of CH flour doughs can be explained. These results agree with those reported previously for different chestnut flour doughs (Moreira et al., 2010). 3.3. Differential scanning calorimetry Fig. 2 shows the thermograms of the tested flour doughs divided into two temperature ranges (a, from 40 to 115 ◦ C and b, from 115 to 150 ◦ C) in order to observe clearer the peaks. All samples showed the typical gelatinization endotherm, G, which appears at relative low temperature (66.7 up to 69.1 ◦ C, Table 3). This endotherm at high water content mainly corresponds to the gelatinization of amylopectin (Russel, 1987; Liu et al., 2006). When available water is restricted, gelatinization can be partly postponed to higher temperatures due to the melting of the remaining amylopectin crystallites (Shogren, 1992) giving as result M1 peak. At intermediate water content, M1 peak can appear as a shoulder overlapped with the G peak giving as result a broad temperature range of gelatinization. At lower moisture content, M1 is clearly separated from G and shifted to higher temperatures up to disappear at dry conditions (Liu et al., 2006).

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In the maize flour doughs, YM flour dough showed separately G and M1 peaks and WM and PM flour doughs only one peak (G + M1). This result agrees with the explained trend of both peaks with water content due to YM flour doughs is the sample with lower hydration (63% w/w) while WM and PM hydration is higher than (80% w/w). CH flour dough also showed a peak with shoulder (G + M1), in spite of it is the sample with the lowest hydration (53% w/w). This result reveals that the different nature of starch (average molecular weights of amylose and amylopectin, amylose–amylopectin ratio, etc.) and its interactions with other hydrophilic components (mainly carbohydrates, protein and fibre) modify the available water for starch gelatinization. Consequently the thermal behaviour of the dough is modified and the temperature ranges for each starch thermal transition must be experimentally determined for each starchy material. Samples with peaks overlap showed a broad temperature range for the glass transition (from 66.7 to 93.3 ◦ C, Table 3). M1 peak of YM flour dough took place between 83.0 and 100.7 ◦ C that agrees with reported data for maize starch at similar hydration (52% w/w) (Liu, Yu, Chen, & Li, 2007). Gelatinization enthalpy is related to the proportion of ordered/disorder material in the starch (Bogracheva et al., 2002). The sum of the associated enthalpy values of G and M1 peaks varied for maize flour doughs between 4.2 and 9.2 J/g and they are in the range of previously reported data (Liu et al., 2007). In the case of CH flour dough the enthalpy value was higher (11.2 J/g) and also is according to those reported for chestnut starch (Cruz, Abrao, Lemos, & Nunes, 2013). Other transition at higher temperature, denominated M2, was observed in all flour doughs. This transition varied between 94.6 to 122.2 ◦ C and corresponds to the melting of amylose–lipid complexes (Biliaderis, Page, Maurice, & Juliano, 1985; Jovanovich & ˜ Anón, 1999). Enthalpy values (from 0.6 up to 1.7 J/g) are lower than the transitions at lower temperatures and the range are also according to bibliography (Liu et al., 2007). No clear relationship was found between enthalpy values and lipid or total starch or damaged starch content. The required energy for the melting of amylose–lipid complexes depends rather on microstructural characteristics of both biopolymers and the accessibility among them.

100 90

Water absorption, % d.b.

80 70 60 50 40 30 20 10 0 0

5

10

15

20

25

Damaged Starch, % d.b. Fig. 1. Relationship among damaged starch and water absorption of tested flour doughs.

30

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a YM

Heat Flow (W/g)

WM

G M1

PM

M2

G + M1 M2

CH

G + M1

M2 M2

G + M1 0.01 W/g 45

55

65

75 85 Temperature (ºC)

95

105

115

b

YM

Heat flow (W/g)

M3

M3

WM

M3

PM

CH

M3

0.05 W/g 115

120

125

130 Temperature (ºC)

135

140

145

Fig. 2. DSC thermograms for yellow (YM), white (WM) and purple (PM) maize flour doughs and chestnut (CH) flour doughs. (a) Temperatures from 45 up to 115 ◦ C, (b) temperatures from 115 up to 145 ◦ C.

Fig. 2b shows the appearance of a new peak, M3, at high temperatures (from 129 to 147 ◦ C) corresponding to the melting of amylose (Liu et al., 2007). This endotherm sets in a narrow interval of temperatures (<7 ◦ C) and in maize flours the peak temperatures showed an inversely relationship with hydration of flour. So, M3 for PM flour dough was into 129.0–132.5 ◦ C and for YM flour dough was 140.1–146.4 ◦ C. This thermal trend with water absorption was also observed in high amylose maize starches (Liu et al., 2005). M3 peak of CH flour dough was shorter and at intermediate temperature (134.6–136.0 ◦ C) in spite of its low hydration, indicating the importance of the amylose nature (molecular size, presence of polymorphs, etc.) on its thermal behaviour. Enthalpy values of M3 for maize flours varied between 6.6 J/g for YM and

2.9 J/g for PM. This result reveals a relationship between hydration and damaged starch proportion and enthalpy values of M3 endotherm. The presence of high proportion of damaged starch gives as result high hydration level to achieve the target consistency and leached amylose interacts easily with other biopolymers and undergoes thermal stress. At low hydration level, amylose is preserved and the corresponding melting heat is higher. CH flour doughs showed an intermediate value for the enthalpy value of M3 endotherm regarding to maize flours. The total enthalpy, HG+M1+M2+M3 , calculated as the sum of the individual enthalpy values of the determined peaks is shown in Table 3. Average enthalpy value for maize flours was 11.5 ± 2.2 J/g and no significant differences were observed between them. Other

R. Moreira et al. / Carbohydrate Polymers 127 (2015) 160–167 Table 3 Onset (To ), peak (Tp ) and final (T1 ) temperatures and enthalpy, Hi , of thermal starch transitions determined by DSC for tested maize and chestnut flour doughs.a

G

M1

M2

M3

To (◦ C) Tp (◦ C) T1 (◦ C) HG (J/g starch) To (◦ C) Tp (◦ C) T1 (◦ C) HM1 (J/g starch) To (◦ C) Tp (◦ C) T1 (◦ C) HM2 (J/g starch) To (◦ C) Tp (◦ C) T1 (◦ C) HM3 (J/g starch) HG+M1+M2+M3 (J/g starch)

YM

WM

PM

CH

66.7a 74.0a 82.6a 2.9a 83.0 91.7 100.7 2.0 100.7c 109.6c 117.6b 0.6a 140.1c 140.2c 146.4d 6.6b 12.1b

66.7a 76.7b 90.6b 4.2a – – – – 94.6a 102.1a 112.0a 1.1a 130.2a 130.3a 137.9c 3.7a 9.0a

68.2b 78.8c 93.3c 9.2b – – – – 97.8b 105.0b 112.4a 1.1a 129.0a 129.1a 132.5a 2.9a 13.2b

69.1b 77.5b 90.2b 11.2c – – – – 102.8d 114.0d 122.2c 1.7a 134.6b 134.7b 136.0b 3.5a 16.4c

a Standard deviations of temperature data were ±0.2 ◦ C and enthalpy data ±0.3 J/g. Yellow (YM), white (WM) and purple (PM) maize flour doughs and chestnut (CH) flour doughs. Data value with different letters in rows are significantly different, P ≤ 0.05.

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authors found a proportional trend between hydration and total enthalpy (with values in the same range), but this result was not found in the tested samples (Liu et al., 2006). Total enthalpy (16.4 ± 0.2 J/g) for CH flour doughs was significantly higher, revealing again the specific features of starch from different sources. CH sample showed a differenced behaviour regarding to maize flour doughs at higher temperatures (>135 ◦ C). A broad endotherm among 138 and 168 ◦ C appears corresponding to the melting of sugars (mainly sucrose) of chestnut (data partially shown), Fig. 2b. This peak was not observed in the maize flour doughs due to the low sugar content of the samples. 3.4. Dynamic thermo-mechanical analysis Fig. 3a shows the elastic modulus peaks during heating of YM and WM dough samples, as example of two different behaviours observed. At low temperatures G values decreases slightly with increasing temperature up to achieve a minimum. This point is labelled like T  o , Table 4, because determines the beginning of the physical phenomena that take place during starch gelatinization, mainly the swelling of the starch granules, Fig. 3a. G increases due to the growing turgor of starch granules. In the tested flour doughs, elastic modulus values sharply increase from 50 to 57 ◦ C, Table 4. This point was not detected during DSC experiments, because

Fig. 3. DMTA rheograms for yellow (YM), white (WM) and purple (PM) maize flour doughs. (a) G from temperatures from 45 up to 115 ◦ C, (b) tan ı for temperatures from 120 up to 160 ◦ C.

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Table 4 Onset (To ), peak (Tp ) and final (T1 ) temperatures of thermal starch transitions determined by DMTA following the elastic modulus (G ) and the damping factor (tan ı) for tested maize and chestnut flour doughs.a YM To (◦ C)

G

To (◦ C) Tp (◦ C) T1 (◦ C) M1

To (◦ C) Tp (◦ C) T1 (◦ C)

M2

◦

To ( C) Tp (◦ C) T1 (◦ C)

M3

◦

To ( C) Tp (◦ C) T1 (◦ C)

a

G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı G tan ı

53.6 ± – – 66.9 ± 74.4 ± 77.0 ± – – – – – – 102.5 ± – 106.9 ± – 113.7 ± – 118.9 ± – – – – – – –

WM 0.1a

0.1a 0.1a 0.2a

2.1 0.5b,c 0.1c 0.8a

50.0 ± – – 68.6 ± 74.5 ± 75.8 ± 86.8 ± 83.0 ± – – – – – – 99.1 ± – 103.7 ± – 111.4 ± – – 121.9 ± – 131.2 ± – 138.2 ±

PM 1.3a

1.9a 0.3a 2.0a 0.2a 0.2a

1.2a 0.2a 2.8a

2.5 2.7 2.7

56.1 ± – – 71.5 ± 77.3 ± 79.0 ± 85.0 ± 87.5 ± – – – – – – 102.8 ± – 108.5 ± – 111.7 ± – – 126.4 ± – 136.8 ± – 146.2 ±

CH 0.2a

0.6a 0.5b 0.6a 0.4a 1.5a

0.1a,b 0.5b 2.4a

56.9 ± – – 71.0 ± 76.9 ± 78.9 ± 86.5 ± 84.5 ± – – – – – – 109.1 ± – 114.9 ± – 121.8 ± – –

2.5a

0.7a 0.2b 0.6a 0.3a 0.1a

0.4c 0.6c 1.3a

1.5 1.2 0.1

– – – –

Yellow (YM), white (WM) and purple (PM) maize flour doughs and chestnut (CH) flour doughs. Data value with different letters in rows are significantly different, P ≤ 0.05.

involved processes do not modify significantly the thermal properties of the system. Nevertheless, starch gelatinization continues at higher temperatures with the disintegration of the granules and starchy polymers melting with a generation of a continuous matrix of leached amylose molecules that increases the viscosity and consequently the viscous character of the multi-phased system (Ring, 1985). This aspect is confirmed by the analysis of the damping factor, tan ı. This factor increases below the temperature corresponding to the onset temperature of gelatinization, To , measured by DSC up to a maximum value, Tp . The peak temperature evaluated by the damping factor coincides with the peak temperature, Tp , determined by means of the relative maximum of G , Table 4. It can be observed, Tables 3 and 4, a good agreement between onset and peak temperatures measured by DSC and DMTA with deviation less than 2 ◦ C between them in all tested samples. It is noteworthy that the tan ı peaks are relatively small, because the starch transitions are more limited by the presence of other hydrophilic polymers in the dough together with the restricted amount of water and a significant fraction of starch crystallites melt at higher temperature (Xie, Yu, Chen, & Li, 2008). Furthermore, the presence of amylose helps to maintain starch granule integrity during gelatinization and more transitions can be observed at higher temperatures (Debet & Gidley, 2007). Final temperature, T1 , can be evaluated through the minimum value of tan ı and also by the point in which the slope (straight line in Fig. 3a) of G changes after Tp . Indeed, this first peak corresponds, as it was explained during discussion of the DSC results, to the addition of two transitions G and M1 for WM, PM and CH flour doughs while YM sample showed separated peaks. During DMTA both peaks of YM flour dough are not observed, but a broader temperature interval with constant slope is obtained. These results indicate that structural changes, phase transitions and rearrangements promoted during G and M1 entdotherms measured by DSC are jointly observed by means of DMTA. Other authors have studied isolated starches with different amylose to amylopectin ratio and, in the case of starches with a normal ratio (0.15–0.25), peaks in G and tan ı are reliably obtained (Warren et al., 2012). These authors also found, using these kind of starches and with excess of water, good agreement among G peak and Tp

evaluated by DSC. Nevertheless, they found that Tp measured by tan ı was in agreement with To measured by DSC, result that was no observed in the tested samples in this work. Nevertheless, these results are not rigorously comparable because flour doughs are complex materials and with restricted water accessible the peaks are shifted at higher temperatures and occur into a broader temperature interval. M2 and M3 peaks determined by DSC are observed by means of the evolution of G or tan ı with temperature. Specifically, M2 peak is difficult to observe because is coincident with the water evaporation from the dough sample. In these circumstances, simultaneous processes that affect to mechanical properties take place and the elastic modulus data show some dispersion due to the important physical and structural changes (fracturing by the rupture of the matrix porosity) promoted by the water removal (Jefferson, Lacey, & Sadd, 2007). Furthermore, G passes through a minimum value that is labelled like peal temperature of M2 peak, Tp . To and T1 temperatures were determined by means of slope changes of G before and after Tp , respectively, with increasing temperature. The peaks took place in a narrow interval of temperatures varying from 99 to 111 ◦ C for WM flour dough and from 109 to 122 ◦ C for CH flour doughs. It was observed that To and Tp determined by DMTA corresponding to M2 peak shifted to higher temperatures than those measured by DSC. This result indicates that the melting of amylose–lipid complexes is accompanied with an elastic modulus decrease, depending on moisture content of the dough because this transition is retarded when water content diminishes. Nevertheless, the ends of the M2 transition, T1 , were the same by means of both methods for the tested flour doughs, Tables 3 and 4. Above M2 transition, G values increase sharply during baking by the complex phenomena related to the crust formation that give as result a more rigid and stiff material. Regarding to M3 peak, corresponding to the melting of amylose, it can be monitored by tan ı by the following points. Onset temperature corresponded to the slope increase of the tan ı curve, Tp was the temperature with the maximum value of tan ı and T1 with a later change of the slope (Fig. 3b). To varied between 121.9 and 126.4 ◦ C and T1 from 138.6 and 146.2 ◦ C for WM and PM doughs, respectively. Peak

R. Moreira et al. / Carbohydrate Polymers 127 (2015) 160–167

temperature varied from 131.2 to 136.8 ◦ C for the tested doughs. It is noteworthy that in the YM and CH dough no M3 peak was observed in tan ı curve in this interval of temperatures. This result can be related to lower hydration levels of both doughs. M3 peaks determined by DMTA are broader than those obtained by DSC due to the associated structural changes and re-arrangements of the transitions take place in an extensive range of temperatures. 4. Conclusions Starch transitions that take place on flour doughs like gelatinization or amylose melting depend on starch nature. Some of these processes can be determined by DMTA (e.g. amylose melting analyzing tan ı curve) even in a clear manner compared to DSC (e.g. gelatinization, following G curve changes). Moreover, DMTA allows the determination of processes that do not modify the thermal properties of dough and consequently cannot be determined by DSC (e.g. swelling of the starch granules). At temperatures above 100 ◦ C all processes determined by DMTA are retarded compared to DSC, due to water content diminution by evaporation during the essay. Consequently, the results obtained show that DMTA is a valuable experimental technique to evaluate the starch transitions during starchy products processing. Acknowledgements The authors acknowledge the financial support to Ministerio de Economía y Competitividad of Spain and FEDER (CTQ 201343616/P). References AACC. (1994). Approved methods of the American association of cereal chemistry. St. Paul: American Association of Cereal Chemists. AACC. (2000). Approved methods of the American association of cereal chemistry. Method 76.13. St. Paul: American Association of Cereal Chemists. AOAC. (1995). Official methods of analysis of AOAC International. Washington: Association of Official Analytical Chemists. AOAC. (1996). Official methods of analysis of AOAC International. Washington: Association of Official Analytical Chemists. Biliaderis, C. G., Page, C. M., Maurice, T. J., & Juliano, B. O. (1985). Thermal characterization or rice starches: A polymeric approach to phase transitions of granular starch. Journal of Food Agricultural and Food Chemistry, 34, 6–14. Bogracheva, T. Y., Wang, Y. L., Wang, T. Y., & Hedley, C. L. (2002). Structural studies of starches with different water contents. Biopolymers, 64, 268–281. Bolade, M. K., Adeyemi, I. A., & Ogunsua, A. O. (2009). Influence of particle size fractions on the physicochemical properties of maize flour and textural characteristics of a maize-based nonfermented food gel. International Journal of Food Science and Technology, 44, 646–655. Chanvrier, H., Appelqvist, I. A. M., Li, Z., Morell, M. K., & Lillford, P. J. (2013). Processing high amylose wheat varieties with capillary rheometer: Structure and thermomechanical properties of products. Food Research International, 53, 73–80. Cruz, B. R., Abrao, A. S., Lemos, A. M., & Nunes, F. M. (2013). Chemical composition and functional properties of native chestnut starch (Castanea sativa Mill.). Carbohydrate Polymers, 94, 594–602. Debet, M. R., & Gidley, M. J. (2007). Why do gelatinized starch granules not dissolve completely? Roles for amylose, protein, and lipid in granule ghost integrity. Journal of Agricultural and Food Chemistry, 55, 4752–4760. Eliasson, A. C. (1980). Effect of water content on the geletanization of wheat starch. Starch/Stärke, 32, 270–272.

167

French, D. (1984). Organization of starch granules. In R. Whistler, J. N. BeMiller, & E. F. Paschall (Eds.), Starch: Chemistry and technology (pp. 183–256). Orlando: Academic Press. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards, Section A, 89–102. Hasjim, J., Srichuwong, S., Scott, M. P., & Jane, J. (2009). Kernel composition, starch structure, and enzyme digestibility of opaque-2 maize and quality protein maize. Journal of Agricultural and Food Chemistry, 57, 2049–2055. ICC. (1996). ICC-standard methods. Vienna: International Association for Cereal Chemistry. ICC. (2008). ICC-standard methods. Vienna: International Association for Cereal Chemistry. ISO. (1982). ISO 7302, cereals and cereal products. Determination of total fat content. Geneva: International Organization for Standarization. Jang, J. K., & Pyun, Y. R. (1996). Effect of moisture content on the melting of wheat starch. Starch/Stärke, 48, 48–51. Jefferson, D. R., Lacey, A. A., & Sadd, P. A. (2007). Crust density in bread baking: Mathematical modelling and numerical solutions. Applied Mathematical Modelling, 31, 209–225. ˜ Jovanovich, G., & Anón, M. C. (1999). Amylose–lipid complex dissociation: A study of the kinetic parameters. Biopolymers, 49, 81–89. Liu, H., Yu, L., Chen, L., & Li, L. (2007). Retrogradation of corn starch after thermal treatment at different temperatures. Carbohydrate Polymers, 69, 756–762. Liu, H., Yu, L., Xie, F., & Chen, L. (2006). Gelatinization of cornstarch with different amylose/amylopectin content. Carbohydrate Polymers, 65, 357–363. Liu, H., Xie, F., Chen, L., Yu, L., Dean, K., & Bateman, S. (2005). Thermal behaviour of high amylose cornstarch studied by DSC. International Journal of Food Engineering, 1, 3–8. Lund, D. (1984). Influence if time, temperature, moisture, ingredients and processing conditions on starch gelatinization. Critical Review of Food Science and Nutrition, 20, 249–273. Malumba, P., Boudry, C., Roiseux, O., Bindèlle, J., Beckers, Y., & Béra, F. (2015). Chemical characterisation and in vitro assessment of the nutritive value of co-products yield from the corn wet-milling process. Food Chemistry, 143–149. McGrance, S. J., Cornell, H. J., & Rix, C. J. (1998). A simple and rapid colorimetric method for the determination of amylose in starch products. Starch, 4, 158–163. Moreira, R., Chenlo, F., & Torres, M. D. (2011). Rheology of commercial chestnut flour doughs incorporated with gelling agents. Food Hydrocolloids, 25, 1361–1371. Moreira, R., Chenlo, F., Torres, M. D., & Prieto, D. M. (2010). Influence of the particle size on the rheological behaviour of chestnut flour doughs. Journal of Food Engineering, 100, 270–277. Ring, S. G. (1985). Some studies on starch gelation. Starke, 3, 80–83. Rouille, J., Chiron, H., Colonna, P., Della Valle, G., & Lourdin, D. (2010). Dough/crumb transition during French bread baking. Journal of Cereal Science, 52, 161–169. Russel, L. P. (1987). Gelatinization of starches of different amylose/amylopectin content. A study by differential scanning calorimetry. Journal of Cereal Science, 6, 133–145. Shogren, R. L. (1992). Effect of moisture content on the melting and subsequent physical aging of cornstarch. Carbohydrate Polymers, 19, 93-90. Singh, J., & Singh, N. (2001). Studies on the morphological, thermal and rheological properties of starch separated from some Indian potato cultivars. Food Chemistry, 75, 67–77. Tahir, R., Ellis, P. R., & Butterworth, P. J. (2010). The relation of physical properties of native starch granules to the kinetics of amylolysis catalysed by porcine pancreatic ␣-amylase. Carbohydrate Polymers, 81, 57–62. Torres, M. D., Moreira, R., Chenlo, F., & Morel, M. H. (2013). Effect of water and guar gum content on thermal properties of chestnut flour and its starch. Food Hydrocolloids, 33, 192–198. Torres, M. D., Moreira, R., Chenlo, F., Morel, M. H., & Barron, C. (2014). Physicochemical and structural properties of starch isolated from fresh and dried chestnuts and chestnut flour. Food Technology and Biotechnology, 52, 135–139. Vanin, F. M., Michon, C., Trystram, G., & Lucas, T. (2010). Simulating the formation of bread crust in a DMTA rheometer. Journal of Cereal Science, 51, 277–283. Warren, F. J., Royall, P. G., Butterworth, P. J., & Ellis, P. R. (2012). Immersion mode material pocket dynamic mechanical analysis (IMP-DMA): A novel tool to study gelatinisation of purified starches and starch-containing plant materials. Carbohydrate Polymers, 90, 628–636. Xie, F., Yu, L., Chen, L., & Li, L. (2008). A new study of starch gelatinization under shear stress using dynamic mechanical analysis. Carbohydrate Polymers, 49, 297–305.